When you need lightweight fantasy craziness with ultra-violence, wit and nemeses who just keep on coming back, book after book, your two best options are Eoin Colfer's Artemis Fowl books and Derek Landy's Skulduggery Pleasant novels. The Fowl has gone off the boil (I haven't bothered with the latest one) so that leaves Landy and the Skeleton Detective as possessors of the field. Except Pleasant is stuck in another dimension with a bunch of Gods that appear to have escaped from a Lovecraft story. Which means Valkyrie Cain, who is the real protagonist of this series, is on a rescue mission. So...how good is Skulduggery Vol. IV?
Not as good as its predecessors, I'm afraid. Despite everything Landy throws in to change things up, bring new ideas in and continue foreshadowing Momentous Events Looming in the Future, he doesn't succeed very well. The rescue of the Skeleton is too easy and it's getting difficult to believe the characters are in real jepeordy regardless of the situation after they have got through so many fights only to be doctored up to full health in no time and do it all again. And there weren't really many new ideas, besides the foreshadowing.
Book V is already out and it would seem that there are a few more years of Cain's life to get through yet but, like with Fowl I may not be going along for the whole journey.
All that said, there is nothing glaringly wrong with this novel and I'd buy the next one if I come across it when in the right mood, it's just that Landy should think of burying the Skeleton and writing something fresher...